The present invention relates to friction clutches in general, especially to friction clutches for use in motor vehicles. More particularly, the invention relates to improvements in clutch plates or clutch discs which are used in friction clutches to transmit torque between driving and driven parts and normally have friction linings which come into direct frictional engagement with the driving and driven parts, e.g., with a flywheel which receives torque from the crankshaft of an Otto engine or diesel engine and with a pressure plate which is movable axially toward and away from the flywheel.
It is already known to install several damping units between the input and output components of a clutch plate. For example, East German Pat. No. 57,771 discloses a clutch plate wherein a first damping unit is operative during a first stage of angular movement of the input and output components of a clutch plate relative to each other, and a second or main damping unit is operative between such components during the remaining or second stage of angular movement. Each damping unit comprises a set of coil springs which yieldably oppose angular movements of the input and output elements or members of the respective damping units relative to each other. It is also known to provide the one and/or the other damping unit with a load friction generating device whose parts generate friction in response to angular displacements of the input and output elements or members of the respective damping unit relative to each other. The diagram in FIG. 4 of the East German patent indicates that this reference proposes to use a first damping unit which opposes small relative angular displacements of the input and output components of the clutch plate such as will develop between the crankshaft and the change-speed transmission during idling of the engine in a motor vehicle, and to use a second or main damping unit which is operative when the vehicle is in motion. The construction of the patented clutch plate is such that the first and second damping units comprise common input and output elements or members. The patented clutch plate further comprises two discrete friction generating devices which are intended to achieve a variable frictional damping action. One of these friction generating devices is operative during each and every stage of operation of the first damping unit, and the other friction generating device (which constitutes a load friction generating device) is effective during each and every stage of operation of the second or main damping unit. The second friction generating device cooperates with the springs of the second damping unit. A drawback of the patented clutch plate is that the frictional damping action is constant during each and every stage of operation of the first damping unit as well as during each and every stage of operation of the second damping unit. This affects the versatility of the patented clutch plate so that the latter cannot be used in conjunction with many types of modern internal combustion engines which are of lightweight design and wherein the idling speed is very low so that the fluctuations of rotational speed of certain parts are very pronounced.
German Pat. No. 2,318,908 discloses a clutch plate with a load friction generating device which is designed to become operative during a certain stage of operation of the first damping unit as well as during each stage of operation of the second or main damping unit. As shown in the diagram of FIG. 4 in this patent, the clutch plate is designed to generate an asymmetrical frictional hysteresis regardless of whether the crankshaft drives the input element of the change-speed transmission or vice versa, and such hysteresis is superimposed upon the spring characteristic of energy storing means in the main damping unit. The load friction generating device merely increases the moment while it is effective during the last stage of operation of the first damping unit. The patented clutch plate is capable of reducing noise in the power train between the engine and the change-speed transmission, and such clutch plate also contributes to longer useful life of the friction clutch as well as to the elimination of certain problems which are attributable to undesirable vibrations. However, the clutch plate is still unsatisfactory during certain stages of operation of a motor vehicle, particularly while the engine is idling as well as during the period of transition from operation with the first damping unit to operation with the second or main damping unit. Thus, the patented clutch plate cannot adequately suppress resonant movements (vibrations) and the attendant rattling and/or rumbling noises.